Radio transmission system



oct. 16, 1945.' R M., SPRGU'E mh 2,387,098

-RADIo TRANSMISSION SYSTEM Filed Aug. 12, 1943 lll 11N mbk w sem @MM n o.

m E O W2K/w, n IT A D B 02V.

Patented Oct. 16, 1945 RADIO TRANSMISSION- SYSTEM Robert M. Sprague, Manhasset, and Everett G. l l

Fraim, Hicksville, N. Y., assignors to Press l Wireless, Inc., Chicago, Ill., a corporation of Delaware Application August 12, 1943,l Serial No. 498,278

(Cl. 25o- 9) l multiplied frequency is then amplified in device 1 Claim.

This invention relates to radio telephone and telegraph systems and more especially to systems in which different messages are sent on the same carrier channel-without interference by employing two different kinds' of modulation.

' The invention is in the nature of an improvement on the system disclosed in application Serial No. 478,854, filed March 12, 1943.

A principal object of the invention is to provide improved circuit arrangements for utilizing a single carrier channel for different classes of communication service by means of simultaneous amplitude and frequency modulation.

A feature of the invention relates to a flexible service radio transmitter whereby the same transmitter can be used for transmission of telegraph cr telephone signals by amplitude modulation or keying, and can also be used to transmit simultaneously telephone signalsA and other signals, e. g., facsimile signals, by ytwo different kinds of modulation.

Another feature relates to an improved circuit arrangement for frequency shifting a carrier by converting telegraph signals or facsimile signals, into frequency modulations of a low frequency oscillator which is sufficiently inherently stable without using control crystals, and converting said frequency modulations into radio frequency signals in the form of a frequency shifted radio frequency carrier using a crystal stabilized oscilator.

A'further feature. relates to the novel organization, arrangement and relative interconnection of parts which constitute an improved radio transmitter for simultaneously radiating different kinds of intelligence signals.

Other features and advantages not specifically set forth will be apparent after a consideration of the following detailed descriptions and the appended claims. l n

Referring to the drawing which illustrates one embodiment of the invention, the transmitter comprises a grid-controlled amplifier Itube I of any well-known type for amplifying radio frequency oscillations, e. g., at from two to fourr megacycles. If desired, the tube I may be connected to a crystal control device 2, by means of a suitable switch 3 when the system is to be used as a straight telegraph or telephone transmitter. The output of tube I is fed to a buffer tube 4 and thence to a telegraph keying tube 5 whose conductivity is controlled by any wellknown keying device 6. The keyed radio frequency carrier may then be doubled in frequency in device l, and then redoubled in device 8. The.

9 and in the balanced or push-pull amplifiermodulators I0, I l, the output of which is applied over suitable feeder lines to a radio antenna. When it is desired to use the transmitter for Vvoice frequency or telephone transmission, a suitable sourcer I2 of voice frequency waves is connected 4to the plates of tubes I0 and II to plate modulate the carrier in the well-known manner.

The foregoing represents well-known radio equipment which can be used either for telegraph or voice transmission, but not for both simultaneously.l We have discovered that thesystem can also be used for simultaneous telegraph or facsimile transmissionrand for voice by replacing the crystal control device by the improved carrier frequency shifter as shown. i f

For ythat purpose, an audio frequency tone line I6 is connected to a telegraph or facsimile signal generator I5. The presence of the audio fre'- quency toneon the line may represent one signal condition e. g., marking (or black in the case of facsimile) and the absence of ytone may represent another signal condition e. g., spacingf (or white in the case of facsimile). For a detailed description of one manner of'shif-ting from one to the other signal condition, reference may be had to application Serial No. 424,946, filed December 30, 1941. The audio frequency tone is fed to an audio frequency amplifier I8 Vand is then rectified in device I9, the rectified voltage (negative) being applied to the voltage divider resistorl 20. An adjustable tap 2l connects resistor 20 to the first or control grid 22 of a triple `grid or pentode tube 23. The said control grid is normally biased a predetermined amount in the usual Way by cathode resistor 24 and Aits shunt condenser 25. The suppressor grid 26 is grounded in the usual way. However, the shield grid 21 is connected in a D. C. path to the plate 28. By this arrangement, as the Voltage on grid 22 is varied under control OLrectier I9, the effective interelectrode capacitance between electrodes 2l and 28 is varied because the shield grid varies in potential, whereas in the usual shield-grid arrangement the shield grid is maintained at a constant potential. Y

Theplate of tube 23 is coupled through a very low capacity condenser 29, e. g.,'of the order of .00005 mf. to one end of a tuned oscillatory circuit comprising inductance 30 and adjustable shunt tuning condenser 3 I. Connected to the said tunable circuit is an oscillator generator tube 32 of the double triode type, preferably connected to act as a negative resistance oscillator. For that purpose the two grids are connected in cross coupled feed back relation through condensers 33, 34, to the plates 35, 36.

The variations incapacity impedance of tube 23 are reected through condenser 29, as a variable capacity across the oscillatory circuit, thus producing variations in the frequency generated by tube'322- `AF'or. example, when a signal teneis receivedgover lineA I6,- the oscillator 32 may'have its frequency shifted as much as 0.8 kilocycle above its normal frequency of 199.6 kilocycles. In effect therefore, the carrier from tube 32 shifts between 199.6 and 200.4 kilocycles with a mean frequency of 200 kilocvcles. l

Associated with tube 32 is-another oscillator tube 31 which is controlledby a piezo crystal 33 to gener-ate a high frequency carrier of from 1.8 to 3.8 megacycles, e. g., 2.8 megacycles. The high frequency carrier from tube 31 is fed in like phase through coupling condenser 33 to the rst'grids 40, 4|, of a pair of frequency mixer or converter tubesv 42, 43. The second grids of these mixer tubes are connected inpush-pull relation to the coupling condensersy 44, 45, which are fed from the plates 35, 36. .Thethird grids of the mixer tubes are supplied with steady D. C.. for example 225-volts from the positive terminal of the D, C. power supply through suitable lter resistors and condensers. The plates-v or electron collector electrodes of the two mixer tubes are Vconnected in push-pull relation to the output through the push-pull` Winding 46 which. ishvtuned'by shunt condenserA 41. The secondary 48. of the high frequency output coupling transformer is also tuned by a condenserf49. Preferably also, theprim-ary w and secondary circuits of the coupling system are also coupled by respective coils 50. l'. The Q- of the eoupling'circuit 46-48, 50-5L should be high so that thev undesiredside-band is rejected, for example thelower side band resulting l frornthe mixing. of the crystal frequency ofr 2.8 megacycleswith `the `199.6 to 200.4 kilocycle band from-oscillator 32. To. acertain extent,- the orys.- tal frequency is also-suppressed in the coupling, The 200 k-ilocyclic carrier from tube 32 is-mainly suppressedl by reason of the fact that the tuned circuit is tuned to some frequency 4between 2 and 4 megacycles. In other Words, there is impressed onthe amplifier stage 55.a=high-frequency car-- rier having amean frequency of 3. megacycles. The mean frequency of thiscarrier is shifted plus 40u-cycles to represent one signal condition, and minus 400cycles to represent another signal condition. In the case of facsimile, white shades may be represented by a-carrier of 3.0004 megacycles, and black shades may be represented by 2.9996 megacycles, With intermediate shades representedY bycorresponding intervening carrier frequencies.- This shiftable high frequency carrier iscoupled .through condenser 52 and a para sitioA suppressor unit. comprising high frequency chokev 53 and parallel resistor 54, to the control gridof the amplifier tube which for example may be a type GVG Whose second grid is supplied with positive. D. C. potential through the surge suppressor resistor 56 and parallel high frequency choke 51. The high frequency output of tube 55 is then applied over line 58 to the input of amplifier Tube l is-preferably of a type such aaa .6.15. which amplies the 2.99963,0004 megacycle signal which then acts as a carrier for passage through the tubes 4 to H. This carrier of signal-controlled shiftable frequency is then capable of simultaneous plate modulation in tubes I0 and Il under control of the voice frequency source I2, so that both kinds of intelligence e. g., Voice and telegraph or facsimile can be radiated Vfrom thesameantenna'. It will be understood that when the shifted carrier is vto be multiplied in frequency, as in multipliers 1 and 8, this multiplying factor is appropriately taken into account in selecting the frequency of crystal 38.

At the receiver terminal represented by the block 60, the signals are demodulated by a respective amplitude-modulation radio receiver and a frequency-modulation radio'receiver so that the received demodulated signals can be recorded or reproduced in respective mechanisms. It will be understood that the amplitudemodulation type radio receiver has a response which is-substantially flat over the band determined by the amplitude modulation side bands and the stated frequency shift, and is preferably free from beating or heterodyne-oscillators inthe nal detector stage if the receiver is of the superheterodyne type. The frequency-modulation receiver maybe of the'type which employs a local 'beating oscillator and should be preferabhr so adjusted that the local beating. oscillator produces at least six times the current in the radiofrequency detector as is produced by the incoming signal so that the heterodyne beat note remains substantiallyv unaffected by amplitude modulations received on the same carrier frequency. For a more detailed description of a suitable receiving terminal corresponding to terminal 60, reference may be had to application Serial No. 473,854, filed March 12, 1943, which is to be considered as part of the disclosure of the present application. When it is desired to key the transmitter from a D. C. land line, arelay 6| having a normally open set of contacts 62- is provided. Received D. C. keying signals therefore operate relay 6l to apply a positive D. C. keying potential tothe control grid of tube 23. Preferably, the D. C. plate potential for tube 23- e. g., 150 volts, is supplied from a Voltage regulated source e. g., through-a type 1{IR-150 voltage regulator tube for example.

What we claim is:.

A flexible service radio system comprising, a radio transmitter having an input radio frequency amplifier stage, a'keying control stage con-- nected to the amplifier stage, a frequency multiplier stage connected to said keying stage, a modulator stage connected to said multiplier` stage, a source of voice frequency signals connected -to said modulator stage, a radio frequency, oscillator, a subcarrer oscillator, means to shift the frequency of said subcarrier oscillator between predetermined limits in accordance with telegraph signals and the like, means to modulate said radio frequency carrier by. said shiftable subcarrier, means to select one side-band of the modulated radio frequency carrier, and means to apply saidselected side band asa carrier to said input amplifier.

ROBERT M. `SPRAGUE. EVERETT G. FRAIM. 

